The description of the prior art given below is only indicative and is not meant to be exhaustive.
Nanosilver is a highly potent antimicrobial agent. The state-of-the-art antimicrobial finishes based on nano silver are yellow to dark brown in colour depending upon the silver concentration. When these silver nanodispersions are applied to textile or polymeric substrates, it often results in poor aesthetics in terms of lower Whiteness Index or higher Yellowness Index, which is undesirable primarily for white and pale shade garments or substrates.
The problem of colour is further aggravated as nanosilver dispersions are required to be applied on textiles/substrate at high concentration for effective antimicrobial activity (>80%). The Minimum Bactericidal Concentration (MBC) values for dispersions of silver nanoparticles are in the range of 5-1000 ppm (references given in text below), which is quite high. The silver is a precious metal and its application at higher concentration is commercially undesirable.
Also, the dispersions of nanosilver based finishes available in prior art are non durable. The nanoparticles tend to agglomerate and settle down with time in the dispersion form giving poor shelf life. Several additives used for stabilizing these nanofinishes often impart still deeper colours to the nano dispersion making it undesirable. When applied on substrates they lose their efficacy on repeated washing as the silver nano particles or their agglomerate tends to wash off the substrate. If binders are used, the nanosilver loses its high degree of activity against the bacteria.
The synthesis techniques of silver nanoparticles are categorized into bottom-up and top-down approaches. Some of the important approaches are listed below:
Bottom-up approach can be used with the following methods:
Chemical reduction method. This involves the dissolution of silver salt into a solvent (aqueous or non-aqueous) and subsequent addition of a suitable reducing agent e.g. chemical reduction of silver ion in aqueous solutions or non-aqueous solutions (Maribel G. Guzmán, Jean Dille, Stephan Godet, World Academy of Science, Engineering and Technology 43 2008; Zaheer Khan, Shaeel Ahmed Al-Thabaiti, Abdullah Yousif Obaid, A. O. Al-Youbi, Colloids and Surfaces B: Biointerfaces 82 (2011) 513-517; CHEN Yanming Li, CN1994633; Sun, Rong; Zhao, Tao; Yu, Shuhui; Du, Ruxu, CN 102085574).
Template method (Shinsuke Ifuku, Manami Tsuji, Minoru Morimoto, Hiroyuki Saimoto, and Hiroyuki Yano Biomacromolecules 2009, 10, 2714-2717). This process involves synthesizing a desired material within the pores of a porous membrane.
Electrochemical or ultrasonic-assisted reduction (N. Perkas, G. Amirian, S. Dubinsky, S. Gazit, A. Gedanken, Journal of Applied Polymer Science, Vol. 104, 1423-1430 (2007)). The chemical effects of ultrasound arise from acoustic cavitation, that is, the formation, growth, and when solutions are exposed to strong ultrasound irradiation, bubbles in the solution are implosively collapsed by acoustic fields. Cavitation bubble collapse can also induce a shock wave in the solution and drive rapid impact of the liquid to the surface of the particles.
Photoinduced or photocatalytic reduction (C. C. Chang, C. K. Lin, C. C. Chan, C. S. Hsu, C. Y. Chen, Thin Solid Films 494 (2006) 274-278; Lizhi Zhang, Jimmy C. Yu, Ho Yin Yip, Quan Li, Kwan Wai Kwong, An-Wu Xu, and Po Keung Wong, Langmuir 2003, 19, 10372-10380; Wu Juan, Zhang Hongbin, CN 102198511). It takes very long time (sometimes over 70 hours, R Jin, Y Wei Cao and C A. Mirkin, SCIENCE 294 (2001)). The photoprocess involves surface plasmon excitation, and this feature allows one to tailor the size and shape of the disks by simply varying the irradiation wavelength.
Microwave (MW)-assisted synthesis (K J Sreeram, M Nidhin and B U Nair, Bull. Mater. Sci., Vol. 31, No. 7, December 2008, pp. 937-942). MW provides rapid and uniform heating of reagents, solvents, intermediates, and products. Fast heating accelerates the reduction of metal precursors and the nucleation of the metal cluster, resulting in small nanostructures.
Irradiation reduction (S K Mahapatra, K A Bogle, S D Dhole and V N Bhoraskar, Nanotechnology 18 (2007) 135602). Electron irradiation (electron energy) is a new method of reduction of precursor in a solution to produce nanoparticles.
Microemulsion method (Zhi Ya Ma, Dosi Dosev and Ian M Kennedy, Nanotechnology 20 (2009) 085608). Microemulsion consists of a ternary mixture of water, surfactant and oil or a quaternary mixture of water, surfactant, co-surfactant and oil. Different surfactant, that is, different microemulsion system employed in the fabrication process, silver nanoparticles with different diameters or morphologies are obtained.
Biochemical reduction (M. Sathishkumar, K. Sneha, S. W. Won, C. W. Cho, S. Kim, Y. S. Yun, Colloids and Surfaces B: Biointerfaces 73 (2009) 332-338; K. Kalishwaralal, V. Deepak, S. R. K. Pandian, M. Kottaisamy, S. Barath ManiKanth, B. Kartikeyan, S. Gurunathan, Colloids and Surfaces B: Biointerfaces 77 (2010) 257-262), and so on.
The top-down techniques use silver metal in its bulk form, then, mechanically reduce its size to the nanoscale via specialized methodologies such as lithography (Xiaoyu Zhang, Alyson V. Whitney, Jing Zhao, Erin M. Hicks, and Richard P. Van Duyne, Journal of Nanoscience and Nanotechnology Vol. 6, 1-15, 2006,) and laser ablation (A. Pyatenko, K. Shimokawa, M. Yamaguchi, O. Nishimura, M. Suzuki Appl. Phys. A, 79, 803-806 (2004)).
Foremost among all of the above processes is the chemical reduction method that allows production of large quantities of nanoparticles in relatively short periods of time. The other processes are complex and/or require expensive controls and/or infrastructure.
It was observed that with time or high storage temperature, the particles tend to grow or aggregate to form large particles. Coalescence of the nanoparticles may lose their characteristic properties. Thus, stability of the nanoparticles in dispersion is a matter of concern for long time use and to achieve the same efficacy. Antonio M. Brito-Silva et. al., Journal of Nanomaterials, 2010, Article ID 142897, reported synthesis of silver nanoparticles by laser ablation in preformed colloids in non-aqueous media of methanol, acetone, ethylene glycol etc. The stability could be achieved from 8 days till a maximum of around 5 months with different protective agents in non-aqueous media. Some have tried to see the effect of different protective agents on aggregation behaviour of silver nanoparticles and its antimicrobial activity (L Kvitek, A Panacek and J Soukupova, J. Phys. Chem. C 112 5825 (2008); J Soukupova, L Kvitek et al., J Materials Chemistry and Physics 111 77 (2008)). It was observed that addition of ionic protective agents improved the zeta potential (stability) of the nanoparticle dispersion than without protective agents. However, the use of ionic surfactants, which gave the best results, could improve the stability of nanosilver dispersion to only a limited period and antimicrobial activity (MIC value) to 1 ppm.
The antimicrobial activity of silver nanoparticles may be evaluated either in dispersion form to give MBC/MIC values in ppm (μg/ml of dispersion) or after application on substrates in % reduction of microbial growth for a given concentration of silver in ppm (μg/g of fabric) using standard methods such as AATCC100, ASTM E 2149.
The literature reports Minimum Bactericidal Concentration (MBC) for dispersions of silver nanoparticles against pathogenic bacteria to human is in the range of 2-100 ppm for spherical shape. One of the prior art showed 6.7 ppm of 25-50 nm silver nanoparticles against S. aureus and 2 ppm against s. epidermidis using reducing agent saccharides maltose (Ales Panacek, Libor Kvitek, Robert Prucek, Milan Kolar, Renata Vecerova, Nadezda Pizurova, Virender K. Sharma, Tatjana Nevecna, and Radek Zboril, J. Phys. Chem. B 2006, 110, 16248-16253). Another prior art reported an average particle size of 18 nm of spherical nanosilver and MBC values in the range from 10 to 0.15 μg/ml (ppm) against various bacteria that are pathogenic to lower animals such as fish. However, the MBC values were evaluated after 30-90 minutes of incubation time, which is a very short time to see the actual growth of pathogen and is not a standard procedure to evaluate MBC. (Soltani, M., Ghodratnema, M., Ahari, H., Ebrahimzadeh Mousavi, H. A., Atee, M., Dastmalchi, F., Rahmanya, J., Int. J. Vet. Res. 3, 2:137-142, 2009). In another paper, (Ansari M A, Khan H M, Khan A A, Malik A, Sultan A, Shahid M, Shujatullah F, Azam A, Biology and Medicine, Vol 3 (2) Special Issue: 141-146, 2011).
In another paper, (Ansari M A, Khan H M, Khan A A, Malik A, Sultan A, Shahid M, Shujatullah F, Azam A, Biology and Medicine, Vol 3 (2) Special Issue: 141-146, 2011) MBC value of 12.5 to 100 μg/ml (ppm) have been reported towards Staphylococcus aureus, methicillin-sensitive S. aureus (MSSA), and methicillin-resistant S. aureus (MRSA) were examined against commercially available nanosilver particles (5-10 nm particle size).
Sukdeb Pal et. al. (Sukdeb Pal, Yu Kyung Tak, 5 Joon Myong Song, Appl. Environ. Microbiol, 2007 March; 73(6): 1712-1720) have done comparative study on bactericidal properties of different shaped silver nanoparticles with E. Coli. They have shown MIC value for truncated triangular silver nanoparticle to be 1 μg (or 1 ppm), for spherical 50-100 μg (or 50-100 ppm) and for rod shaped particles >100 μg (or >100 ppm).
Antimicrobial activity of silver nanoparticles on textile substrates have also been reported in several studies. For effective antimicrobial activity (>80%) on textile substrates, finishes based on silver nanoparticles are applied on textiles/substrates in concentrations from 5 ppm (Hee Yeon Ki, Jong Hoon Kim, Soon Chul Kwon, Sung Hoon Jeong, J Mater Sci (2007) 42:8020-8024) to 350 ppm (Kanokwan Saengkiettiyut, Pranee Rattanawaleedirojn and Supin Sangsuk, J. Nat. Sci. Special Issue on Nanotechnology (2008) Vol. 7(1)), 75), and even as high as 1000 ppm (Kanokwan Saengkiettiyut, Pranee Rattanawaleedirojn and Supin Sangsuk, J. Nat Sci. Special Issue on Nanotechnology 7 75 (2008).
The silver is a precious metal and its application at higher concentration is commercially undesirable. The result of such a high concentration application eventually gives the fabric yellow to brown tinge depending on concentration. Durability of the silver nanoparticle finish is also a concern. Silver nanoparticles tend to wash off during repeated washing. And if the Ag nanoparticles are used with binders, though wash durability improves to some extent, the maximum efficiency/antimicrobial activity of nanoparticles gets reduced due to hindrance of binder.
In order to overcome the yellowing nature of dispersion of nanosilver, when applied as finish on textile fabrics, nanosilver dispersion of colours other than yellow and brown such as blue or purple can be prepared using the knowledge available in prior art. This can be done either in a solvent (non aqueous) or water (aqueous) system. Most of the processes for different coloured silver dispersions have been prepared in non-aqueous media, which is toxic, not ecofriendly, expensive, and inappropriate for application on variety of substrates such as textile, sport goods, biomedical material, etc.
Only a few studies have been successful in producing such coloured nanosilver dispersion using aqueous system. These often involve preparation by light irradiation reduction method, which is a slow process (Shih-Hong Ciou, Yi-Wei Cao, Huai-Cing Huang, De-Yan Su, and Cheng-Liang Huang, J. Phys. Chem. C 2009, 113, 9520-9525; Lakshminarayana Polavarapu, Qing-Hua Xu, Mohan S. Dhoni and Wei Ji, APPLIED PHYSICS LETTERS 92, 263110, 2008; Jing An, Bin Tang, Xianliang Zheng, Ji Zhou, Fengxia Dong, Shuping Xu, Ye Wang, Bing Zhao, and Weiqing Xu, J. Phys. Chem. C 2008, 112, 15176-15182) gives silver nanodispersions with mixtures of different shape and size particles and three main plasmonic peak were observed at 331, 482 and 661 nm (Bin Tang, Shuping Xu, Jing An, Bing Zhao, and Weiqing Xu, J. Phys. Chem. C 2009, 113, 7025-7030) with low stability of 2-3 months (Bin Tang, Jing An, Xianliang Zheng, Shuping Xu, Dongmei Li, Ji Zhou, Bing Zhao, and Weiqing Xu, J. Phys. Chem. C 2008, 112, 18361-18367) under normal environmental condition. In aqueous based systems, when reduction is done via chemical reduction technique, mainly bigger triangular nanoparticle in the range of 35-200 nm mixed with other shaped particles (Isao Washio, Yujie Xiong, Yadong Yin, and Younan Xia, Adv. Mater. 2006, 18, 1745-1749; Xiaomu Wu, Peter L. Redmond, Haitao Liu, Yihui Chen, Michael Steigerwald, and Louis Brus, J. AM. CHEM. SOC. 9 VOL. 130, NO. 29, 2008; Sihai Chen and David L. Carroll, Mat. Res. Soc. Symp. Proc. Vol. 775, 2003).) has been observed.
Document “Rapid thermal Synthesis of Silver nanoprisms with Chemically Tailorable Thickness”, Advanced Materials, 17, No. 4, Feb. 23, 2005, By Gabriella S. Métraux and Chad A. Mirkin discloses Silver Nanoprisms prepared by a thermal synthetic route resulting in silver nanoprisms with unimodal size distribution. The final colour of the colloidal solution ranged from pink/purple to Turquoise depending on the concentration of NaBH4 used. However, the blue colour colloids were mixture of both prisms (triangular) and spherical shaped particles because the prisms were grown starting from yellow spherical particles as in irradiation method. UV spectrum of blue coloured colloids had three peaks related to the Plasmon bands created with triangular prisms. Also the particle size of silver nanoparticles was large in the range of 20 nm to 50 nm. Though antimicrobial activity of these colloids were not reported, based on other studies as reported above, it is likely that these colloids would be needed to be applied in large concentrations for effective antimicrobial activity due to their large particles size. The dispersion of nanosilver prepared in this prior art was also light sensitive and was stable for only a few months when stored in dark.
Thus, there arises a need to develop antimicrobial finishes based on nano silver that have higher stability, wash durability, antimicrobial activity, and also, that act as a brightening/bluing agent for retaining whiteness of or brightening the white and pale coloured substrates.
The present invention solves the problems of yellowing of fabric and other substrates on application of nanosilver finishes, stability of the aqueous dispersion of nanosilver during storage, their application at low concentrations, and wash durability of nanosilver finish on application. The aqueous dispersion of silver nanoparticles of the present invention is blue in colour, has nanosilver particles of very small in size, and can combine the effect of antimicrobial finish and brightening/bluing agent used for brightening the white and pale coloured substrates. It shows 99.99% antimicrobial activity at very low concentrations, is easy to synthesize directly in aqueous media, and is stable on storage even at high temperature and/or in light. The dispersion stability of the nano silver particles of present invention is for 15 to 24 months. The particles provide wash durability after application on textile, binding with simple heat treatment at temperatures greater than 120 degree C. (120 to 150 degree C.) or with binders at room temperature. The particles and their dispersions have very high compatibility with binders and surfactants of various types.